New `JniFieldDecl` AST struct (name + field_type); `JniClassMember`
gains a `field` variant. After consuming a member-name identifier
in the body loop, the parser branches on the next token: `:` →
field path (parse type expr + `;`), `::` → method path (existing).
`static` fields aren't part of the grammar yet and error explicitly
("static fields not yet supported"); only instance fields land here.
Lowering to JNI `Get<Type>Field` / `Set<Type>Field` arrives in 2.13.
124/124 examples green.
`Point :: #jni_class("...") { x: s32; y: s32; }` should parse,
today's 2.4 body loop sees the identifier `x`, expects `::`, hits
`:` and errors. The make-green follow-up adds a `field` variant to
`JniClassMember` and a parser branch that detects `<ident>:` (vs
`<ident>::`) as the field-decl indicator.
